The development of larger and more powerful wind turbines has required an increase in the dimensions of the wind turbine rotor and the consequential lengthening of wind turbine blades. Such a lengthening of the blades entails an increase in their rigidity, which usually implies the use of carbon fiber or fiberglass based laminated materials.
The inside of a wind turbine blade normally comprises a structural element that gives the blade rigidity. The structural element is enclosed between two shells that constitute the outer structure of the blade and give the blade its aerodynamic shape.
The structural element is normally a box beam, i.e., a hollow rectangular beam that narrows progressively from the blade root to the tip. The box beam comprises two wider walls (caps), which are designed for placement perpendicular to the wind direction, and two narrower walls (webs) on each side of the caps.
The lamination process during blade manufacturing is done manually and thus the total absence of defects, such as the formation of wrinkles while applying laminate layers of material, cannot be ensured easily. These defects could result in the appearance of weaker spots along the blade box beam that are incapable of withstanding the established design loads.
When the wind turbine is in service, given the highly elevated loads that the blades undergo, the presence of weakened areas on the blade raises the likelihood of cracks stemming from material fatigue that could, in the most extreme cases, even cause the blade to fracture.
The sole known method in the state of the art for repairing defects of this sort on the blade box beam entails replacing the entire affected laminate. This method is highly aggressive for the blade, since it entails cutting and burrowing into a section of the shell of up to six meters in length to gain access to and repair the weakened area on the box beam, and subsequently reconstructing the eliminated section of the shell. Given this method's aggressiveness, operations must be done by qualified technicians in environments with controlled conditions. The repair time for such a method is thus long (typically over 300 hours) and consequently expensive.
Moreover, this method has an added inconvenience in that the blade cannot be repaired while mounted on the wind turbine, thus necessitating the additional stages of dismounting the blade, lowering it to the ground level, and moving it to the repair plant, all of which makes the entire process much more expensive.
Documents US 2011/167633 and EP 2 273 102 describe methods for repairing a damaged surface on a wind turbine blade shell. This repair is carried out from outside the blade.
The use of wind turbine blade inspection and cleaning devices is common in the current state of the art. For instance, EP 2 275 670 and DE 10330426 disclose devices that move on the outer part of the blade, and thus their functions are constrained to inspecting the shell, though incapable of accessing inside the blade box beam.
The use of devices adapted for inspecting the inside of a wind turbine blade is also common. DE 102009022179 discloses a device that slides while suspended into a blade pointing towards the ground, and Spanish patent application no. 201100618 (still unpublished on the submission date of the present application) describes an inspection carriage capable of moving along inside the box beam of a wind turbine blade. Nevertheless, none of these documents disclose or suggest a device adapted for repairing a surface on the box beam in the weakened blade area.
It would thus be desirable to have a system, and a method, to reinforce a weakened area of a wind turbine blade quickly and economically, and, in particular, with no need to have to dismount the blade from its service position.